Biology Of Microplitis Mediator (Hymenoptera:Broconidae) And Biological Control Effects To Helicoverpa Armigera (Lepidoptera:Noctuidae) In Xinjiang Cotton Field

Microplitis mediator Haliday (Hymenoptera: Braconidae) is an abundant larval endoparasitoid of cotton bollworm Helicoverpa armigera (Lepidoptera: Noctuidae) in Hebei of China. The biology and mass rearing techniques of M. mediator were conducted in laboratory for producing large numbers of parasitoids efficiently. Mythimna separeta Walker (Lepidoptera: Noctuidae) was used as a host for mass rearing of M. mediator because H. armigera is cannibalistic causing it to be more difficult to rear. Basied on the biology of M. mediator the inundative release experiments were conducted in Xinjiang cotton field to evaluate the biological effects of M. mediator for conrol H. armigera. The results indicated that field release of M. mediator could be used effectively in managing H. armigera in cotton fields in Xinjiang.1, Effects of parasitism by M. mediator on food consumption and development of H. armigera were evaluated in the laboratory. The results showed the significant difference in the food consumption of parasitized larvae and unparasitized larvae by M. mediator. The food consumption of the six-day and the eight-day instar of parasitized larvae were lower by 75.4% and 67.0% respectively than that of the six-day and the eight-day instars of unparasitized larvae. There are significant difference in the food consumption of the six-day and the eight-day instar of parasitized larvae. The food consumption of the six-day instar of parasitized larvae was less than that of the eight-day instars larvae. The food consumption and the weight of parasitized larvae slowly increased 8 or 9 days after being parasitized. M. mediator could suppress the population of H. armigera effectively and reduce the damage caused by the present generation cotton bollworm.2, The preference of M. mediator for larvae of M. separata in different host instars and the host larval suitability for the parasitoid were investigated under laboratory conditions at constant temperature of 25 ± 1℃ and 65% ± 5% RH, 14:10 (L:D)h. When the parasitoid were exposed concurrently to host larvae in the 1^st to 5 instars, they could parasitize the 1^st to 4^th instar larvae, but did not parasitize the larvae in 5th instar; parasitism rates were higher for larvae in the 2^nd and 3^rd instars than in the 4^th and 1^st instars; the preference index of M. mediator for the host larvae in the 1^st, 2^nd, 3^rd and 4^th instar were 0.21, 0.31, 0.27 and 0.21 respectively; The host mortality after exposure were decreased with increase of the host age at exposure to the parasitoid. The parasitoid developed in the 4^th instar larvae had longer development period for the egg-cocoon stage and heavier cocoon weight than in the 3^rd and 2^nd instar larvae. The young M. mediator adults had larger parasitism capacity than the old ones. However, the wasp adult age had no significant effect on the development period for the egg-cocoon and the cocoon weight of the parasitoid in their offspring.3, The effects of M. mediator wasp densities on the percentage parasitism were studied in the laboratory under controlled conditions. 6 days old L. separate larvae were used as hosts. Female M. mediator wasps were exposed at different densities of 1, 2, 4, 6 and 8 to the 50 larvae of L. separata. Totalparasitism was higher at 4 and 6 female M. mediator than other treatment, but the average parasitism of per female M. mediator decreased with the increase of parasitoid density. The cocoons weight and the adult emergence ratio had no difference in different parasitoid density treatments. The sex ratio (F: M) increased with the increase of parasitoid density.4> The effects of temperature and nutrition on the adult longevity of M mediator were examined in both laboratory and field cage experiments. The results indicated that adult longevity had significantly difference under different temperature conditions and decreased with the higher temperature. Honey water can effectively enhance longevity of wasps. The longevity of female wasps was longer than that of male wasps under the same conditions. The adults have the longest longevity of 15.4 d for female and 9.1 d for male in 18°C with 10% honey water supply and the shortest longevity of 1.1 d and 1.0 d in 38°C with no food supply. In field cage experiments the adult longevity was 5.2 d for female and 3.8 d for male in cages with no cotton plants and hosts and 7.6 d for female and 3.9 d for male in cages with cotton plants. After released in cotton field, the percentage cumulative mortality of M. mediator wasps was increased with the released days and reached 91.7% for female and 98.3% for male in the 8th day.5, According to the reproduce coefficient, parasitism to the host and mass rearing method, M. separata was selected as a host for mass-rearing of the parasitoid M. mediator. Comparing with other hosts of M mediator, M. separata is not cannibalistic and therefore economical to mass rear. The inducement and termination methods of diapause cocoons were studed in the laboratory and then the technic flow for mass rearing of M. mediator have been established to produce diapause cocoons continuous. The storage conditions of diapause cocoons and field release methods also have been definited.6, Studies on the dispersal directions and distance of M. mediator wasps in Xinjiang cotton field during the bud and boll stage were conducted in 2003 and 2004. The results indicate that the effective dispersal distance (EDD) of M. mediator was about 15 m and the maximal dispersal distance was 50 m. The wasps were favored to dispersal along the cotton plant rows. The dispersal direction of M. mediator in cotton field was related to wind spreed and direction, when the average wind spreed was lower (1 m/s) the main dispersal direction was up the wind and no difference among the eight directions.7> Field experiments and surveys were conducted in 2003 and 2004 to evaluate the efficiency of using the parasitoid M. mediator to control populations of H. armigera in cotton fields in Northwestern China's Xinjiang Province. The population levels of the second generation H. armigera in Southern Xinjiang in 2003 and 2004 were ca. 20-60% above the economic threshold of this pest. The longevity of field released female and male parasitoids was 7.6 ± 0.4 d and 3.9 ± 0.4 d, respectively. Field tests showed that the number of parasitized H. armigera increased with an increasing number of M. mediator cocoons released in the field. The release of 2,250 or 15,000 laboratory-reared M. mediator cocoons per ha in cotton fields resulted in 38% to 61% parasitism of H. armigera in 2003, and 35% to 66% parasitism in 2004, respectively. The efficiency of the parasitoids varied considerably with different numbers of parasitoid cocoons and with the frequency and sites of release. However, the highest control efficiency in these studies was obtained with a single release of cocoons and the highest number ofrelease sites. Large-area releases of M. mediator in cotton fields of Shufu, Shule and Shache counties resulted in more than 60% parasitism and an 80% decrease in cotton boll and bud damage when compared with the control fields.